The heightfield is generated
from a photograph or image. The image has its own dimensions, but the
mesh usually has other dimensions. You can choose the size of your output
mesh with the following parameters in the main dialog. (The
example shown below is for a rectangular mesh, but PhotoToMesh also does
cylindrical ones.)

If you have 400
points in X and Y then 399 x 399 = 159201 faces will be created in a quad
mesh surface.

In a closed triangular
surface the number of points is roughly quadrupled, about 640,000 faces,
with the example above.

Neither STL nor
DXF have units (inches miles centimeters lightyears). They simply have
numbers. So the Mesh X Size and Mesh Height are pure numbers which
the software you use on the output of PhotoToMesh will interpret according
to the settings you use.

There is a free demo available
which you can use to try out your own image, click on the button below
to download it: